Use last_master_timestamp from mysql_bin_log and not raft log

Summary:
last_master_timestamp in rli->relay_log is not initialized
and not kept up to date. Although this sounds unintuitive from raft
point of view, its not from the non-raft standpoint. In non-raft mode
tailers are always served from binlog files and not from relay log
files and hence keeping mysql_bin_log up to date was understandable.

During the port of Dump log functionality to 8.0, I think by mistake
references to mysql_bin_log in rpl_binlog_sender were migrated to
Dump_log. This however caused a Uninitialized Memory Read issue because
of the uninitialized last_master_timestamp value.

Reviewed By: abhinav04sharma

Differential Revision: D34843214

fbshipit-source-id: a615975a302e13984a2f6d6b0b778213c30061f6

sql/binlog.cc

@@ -4534,6 +4534,7 @@ MYSQL_BIN_LOG::MYSQL_BIN_LOG(uint *sync_period, bool relay_log)
       checksum_alg_reset(binary_log::BINLOG_CHECKSUM_ALG_UNDEF),
       relay_log_checksum_alg(binary_log::BINLOG_CHECKSUM_ALG_UNDEF),
       engine_binlog_pos(ULLONG_MAX),
+      last_master_timestamp(0),
       previous_gtid_set_relaylog(nullptr),
       raft_cur_log_ext(0),
       setup_flush_done(false),

sql/rpl_binlog_sender.cc

@@ -1341,7 +1341,7 @@ int Binlog_sender::send_heartbeat_event(my_off_t log_pos, bool send_timestamp) {
   // value in the HB event otherwise we use now()
   time_t ts = 0;
   if (send_timestamp) {
-    ts = dump_log.get_log(false)->last_master_timestamp.load();
+    ts = mysql_bin_log.last_master_timestamp.load();
     if (ts == 0) ts = time(nullptr);
   }
   int4store(header, ts);